Field of the Invention
The present invention relates to a method for applying a field grading layer to a cable, a cable comprising said field grading layer, a crosslinkable field grading tape, and the use of said method or said crosslinkable field grading tape on a cable termination or a cable joint.
The present invention relates more particularly to a field grading layer applicable for high voltage terminations and factory and repair joints for direct current (DC) applications. These field joints are applicable for instance in subsea connections.
Description of Related Art
Field grading layers are commonly used in alternating current (AC) and direct current (DC) accessories including cables to control the electric field.
To obtain the field grading effect, the field grading layer is made of a material having a resistivity that depends on the strength of the electric field, wherein this dependence can be linear or non-linear.
The material of the field grading layer can comprise an insulating polymer matrix, non-linear or linear conductive elements, and optionally additional conductive elements which do not have field grading properties.
At cable joints/terminations, not only the conductors must be connected but also the field grading layers must be terminated and connected to ground and to the high voltage conductor.
US2010/0147556 describes a field grading material comprising a dielectric polymer matrix such as EPDM rubber (ethylene propylene diene monomer), a plurality of micro varistor particles, and a plurality of other conductive particles referred to as bridging particles. The resistivity of the varistor particles strongly depend on the strength of the electric field, whereas the bridging particles do not have this field dependency but influences the general conductivity of the field grading material. The varistor particles accordingly provide the non-linear properties, whereas the bridging particles provide electrical “bridges” between the varistor particles. The exemplified varistor particles are inorganic particles of SiC and ZnO. Examples of bridging particles include carbon black, and inorganic oxides such as TiO2, antimony doped SnO2. However, US2010/0147556 does not describe the method to prepare the field grading material and apply it to a power cable.
WO 2004/038735 describes a field grading material consisting of a polymeric matrix and an inorganic filler such as ZnO, SiC, Al2O3, TiO2 or BaTiO3. The polymeric matrix of the field grading material consists of polyolefin rubber or thermoplastic polyolefin elastomer/plastomer, preferably including EPDM rubber or silicone rubber. However, the application method is not disclosed in details.
In the cable industry, a common method to prepare a termination or a premolded joint including a resistive control system (i.e. field grading system) is to compound a field grading material comprising at least one elastomer matrix and a conductive filler having field grading properties and then, to mold said field grading material into a premolded joint or termination. The field grading material can also be molded as a tube which can be slipped onto the cable end or joint area. Indeed, low and medium voltage accessories generally include a resistive control system either as separate resistive control tubing, or integrated as resistive control coating in an insulating tubing. However, said methods include at least one molding step which is generally performed at high temperature (e.g. above 200° C. for several hours) and therefore, can give rise to thermal stress on the conductive filler and reduce its field grading properties. In addition, the adhesion and electrical contact of the field grading layer with the insulation of the power cable is not optimized.
Indeed, the adhesion between the field grading layer and the insulation of the power cable relies on pressure and involves the presence of air at the interface between the field grading layer and the insulation, and therefore of partial discharges. To avoid said partial discharges and to enable the field grading layer to slide easily on the power cable, a sliding oil may be used to fill the voids of air at the interface which reduces the adhesion. Further, adhesion and electrical contact have a strong relation and it is wished to have a good electrical contact between the insulation layer and a field grading layer.
Accordingly, it still exists a need for an alternative field grading layer application method which induces less thermal stress on the conductive filler (especially if it is an organic filler) and which improves adhesion and electrical contact to the insulation of the power cable. Indeed, the application of a field grading layer to a cable termination or joint by ensuring physical contact between the insulation of said cable and said field grading layer is of high importance for the transfer of surface charge.